Fix SEGV in _thread_cleanup_author if author ends with ', '

Fix SEGV in _thread_cleanup_author if author ends with ', '

Admittedly, an author name ending in ',' guarantees this is spam, and
indeed this was triggered by a spam email, but that doesn't mean we
shouldn't handle this case correctly.
We now check that there is actually a component of the name (presumably
the first name) after the comma in the author name.

Signed-off-by: Dirk Hohndel <hohndel@infradead.org>